The invention is directed to cold sealable adhesive articles. The present invention relates to a method of preparing adhesive coated articles and cold seal bonded laminates therefrom. Specifically, it relates to a method wherein certain polychloroprene based adhesive compositions are used to form adhesive coated substrates that are non-blocking and pressure sealable at ambient temperature. In a preferred embodiment, the invention relates to a method for the preparation of coated substrates and articles that are particularly useful for the manufacture of medical packaging and tamper evident applications.
It is generally known that water-based “cold seal” adhesives can be used to form dry-bond laminates for protective packaging. The terminology “cold seal” refers to a class of adhesives which possess the ability to form bonds to themselves at  ambient temperature with pressure, yet are essentially tack free to the touch, allowing coated substrates to be wound upon themselves and stored without adhering to the opposing face of the substrate backing. Adhesives of this type have been employed in a variety of packaging applications including food (i.e. flexible packaging for candy wrappers, chips etc.); medical packaging; self-seal and tamper evident envelopes; banding for paper money, napkins, and clothing; and protective packaging such as fold over “blister” packages for hardware and small parts. Additionally, the cold seal adhesives have found utility as anti-skid coatings and for release-paper free tapes such as tab-less diaper tapes.
Natural latex based products have dominated the cold seal market for quite some time, since uncompounded natural latex exhibits properties amenable to cold seal applications. Natural latex based cold seals have several deficiencies. For example, in medical device packaging, natural latex based cold seals contain proteins that have been identified as skin sensitizers. Additionally, natural latex based cold seals often discolor, e.g, yellow, and oxidized upon aging resulting in undesirable aesthetics and reduced bond strength of the coated substrates and articles.
Furthermore, the adhesion characteristics of natural latex based cold seals are often reduced by irradiation, which is required for the sterilization of packaged medical bandages and devices. It is believed that irradiation causes the natural latex to crosslink, significantly changing the bond properties.
For medical device packaging it is often desirable that the adhesive coated article resist yellowing when subjected to heat. Yellowing is often undesirable because the discoloration can diminish the aesthetics of the packaging.
Several references that disclose water-based synthetic cold seals, and their method of use, are described below:
The Bayer Corporation Division of Fibers, Organics, and Rubber provides product literature describing “Dispercoll C Polychloroprene Laticies—General Information On Properties And Compounding For Adhesive Applications”. The polychloroprene latex peel strength, on canvas duck, are evaluated using a room temperature bond method wherein the aqueous polychloroprene latex is coated onto the substrate and air dried. The canvas is then folded onto itself and immediately pressed in platen at 200 psi for 15 seconds.
U.S. Pat. No. 4,889,884 issued to Dust et al., Dec. 26, 1989 claims a cold-seal adhesive composition comprising about 45 to 70% by wt. of a synthetic base polymer and about 30 to about 55% by wt. of a synthetic secondary polymer, wherein the base polymer comprises a synthetic copolymer, a synthetic copolymer, or any mixture thereof characterized in that it possesses a Tg of about −40 to about −60° C. and a Williams plasticity number of about 2.5 to 2.9; and the secondary polymer comprises a styrene-acrylic copolymer, a synthetic styrene-acrylic resin or any mixture thereof, or an admixture of one or more styrene-acrylic copolymers or styrene acrylic resins, or any mixture thereof with one or more synthetic polymer, copolymers, or synthetic resins, characterized in that it possess a Tg of about −35 to about 5° C. and a plasticity number of about 5.0 to about 5.6. At column 2, lines 41-47 it is states that, “Virtually any synthetic polymer meeting these criteria (Tg and Williams plasticity) can be used in the adhesive compositions of this invention, but preferred polymers include acrylic copolymers such as a copolymer comprising 60% (by wt.) butyl acrylate, 30% (by wt.) 2-ethylhexyl acrylate and 105 vinyl acetate; and styrene butadiene rubbers.”
Similarly, U.S. Pat. No. 4,902,370 issued Feb. 20, 1990 as a divisional of the '884 claims a method of forming a cold seal bond on a substrate by applying the recited adhesive, drying said emulsion to form a dried film, and subsequently bringing the coated portion of the substrate in contact with each other such that a bond is effected.
U.S. Pat. No. 5,314,944 issued to Chao, May 24, 1994 relates to a pressure sensitive adhesive that is tack-free to the touch, but forms a permanent bond under moderate pressure. The pressure sensitive adhesive contains a tacky adhesive latex, a non-tacky latex or polymer, and an inert filler. The tacky adhesive latex provides the sealing properties, the non-tacky latex or polymer provides blocking resistance and the filler absorbs oil that may adversely affect the sealing properties. The tacky adhesive latex is generally an acrylic type adhesive latex. The non-tacky latex may be an acrylate latex or a styrene-butadiene latex or a non-tacky polymer such as polyvinyl alcohol, starch and cellulose derivatives.
U.S. Pat. No. 5,567,773 issued to McGee et al., Oct. 22, 1996 is directed to an adhesive coating which is pressure sealable to itself and non-blocking to a dissimilar coating comprising a blend of a) a soft polymer having a measured glass transition temperature below room temperature; and b) a hard polymer having a measured glass transition temperature which is higher than the measured glass transition temperature of the soft polymer in a low blocking, pressure sealable coating formable proportion.
U.S. Pat. No. 4,888,395 issued to Min, Dec. 19, 1989 relates to core/shell polymers containing a rubber core and thermoplastic shell acrylate-vinylacetate-acrylic terpolymer having excellent cold seal strength properties. The core/shell polymer contain about 1 to 50% by weight of a rubber core. At Column 3, lines 54-60, this reference states “Typical rubbers which can make up of the core include polyisoprene, polybutene, polychloroprene (neoprene), poly (1,3-butadiene), natural rubber, butadiene/styrene and the like.” Hence, this reference is also directed to cold seal compositions containing a blend of polymers.
Additionally, aqueous polychloroprenes have been used as an ingredient in contact adhesives. For example, U.S. Pat. No. 5,476,896 issued to Pereira et al. Dec. 19, 1995 relates to a water-based contact adhesive comprising a carboxylated polychloroprene latex which provides elastomeric properties, a carboxylated poly (vinyl acetate-ethylene) aqueous emulsion which provides toughness to the adhesive, a tackifier resin dispersion, an organic water soluble crosslinking agent for the carboxyl functionality of the polychloroprene and poly (vinyl acetate-ethylene); and optionally an adhesion promoter. The compositions exemplified contain at least 10% rosin ester dispersion.
Contact adhesives differ substantially from cold seal compositions. As exemplified in Pereira et al., contact adhesives generally comprise tackifying resins. Whereas cold seal compositions require a proper balance of properties such that the adhesive will form a bond to itself with pressure yet do not exhibit tendencies to adhere to other substrates once dried, contact adhesives may either be permanently pressure sensitive or unable to form a bond without heat activation once dried. Further, contact adhesive typically refers to an adhesive that is coated on at least one substrate and adheres to itself upon contact and drying, whereas cold seal compositions are coated on both surfaces intended to be bonded and the coated surfaces are not contacted until after the adhesive is completely dried.